Fix #1799, don't allow the shot to be taken more than once.
Fix #1799, don't allow the shot to be taken more than once.

mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/pageshot,fzzzy/pageshot,fzzzy/pageshot,mozilla-services/screenshots,mozilla-services/screenshots,mozilla-services/screenshots,mozilla-services/pageshot,mozilla-services/pageshot,mozilla-services/screenshots,fzzzy/pageshot